Numerous electrical components are sensitive to electromagnetic fields. Electrical equipment that operates at radio frequencies is especially sensitive to electromagnetic interference (EMI) from external radio frequency (r.f.) fields. Sensitive equipment is typically housed in an enclosure which shields the equipment from extraneous signals. In addition to equipment which must be protected from external fields, electrical apparatus which produce r.f. signals can also require shielding. Such apparatus can leak r.f., by radiation or conduction, impairing the performance of surrounding devices.
Electromagnetic shielding enclosures are typically fabricated from material which either predominantly reflects or absorbs incident r.f. radiation. Metals such as aluminum reflect r.f., while carbon and/or metal loaded polymers absorb r.f. radiation. Such enclosures vary in size and shape, depending upon the equipment they are designed to house. In order to install the equipment, one or more openings are provided in the enclosures. Tight sealing must be provided around these openings in order to prevent r.f. penetration through gaps. Small gaps act as antennas through which r.f. radiation is transmitted. Generally, the openings are sealed with mechanical fastening members. Threaded fasteners, such as screws, are used as sealing members for numerous r.f. enclosures. While threaded fasteners permit ready access to the equipment inside, they are time-consuming and costly to install. Additionally, it is difficult to ensure a low impedance contact along the joint interface between adjacent fasteners for the lifespan of the seal. As the fasteners are removed and replaced to service the housed equipment, the seal can be degraded, causing gaps which transmit radiation. To prevent leakage, mechanical fasteners must be precisely spaced, the inter-fastener spacing being determined by the necessary degree of attenuation.
For very high attenuation requirements, sealing gaskets are provided around the opening in the space between the joining surfaces. Gaskets help ensure high attenuation since they fill any gaps or surface irregularities. Such gaskets are difficult and expensive to install, and must be replaced each time the r.f. enclosure is opened. Further description of conventional r.f. shielding is described in Hickman, Newnes Practical R. F. Handbook, (Newnes, Butterworth-Heineman, Ltd., Oxford), c. 1993, the disclosure of which is incorporated herein by reference.
Thus, there is a need in the art for improved EMI shielding. In particular, there is a need for improved EMI shielding which permits ready access to equipment housed in an enclosure without the need for removing and replacing numerous mechanical fasteners.